DESCRIPTION(provided by applicant): The overall goal of the proposed research is to investigate the mechanisms of allogeneic stem cell education in patients with severe combined immunodeficiency (SCID) given transplants of rigorously T cell-depleted bone marrow cells without pre-transplant chemotherapy or post-transplant graft-versus-host disease (GVHD) prophylaxis. The PI has administered the latter therapies to 128 such infants at Duke over the past 20.5 years, with an overall survival rate of 78%. However, the immune reconstitution in these patients is not perfect. The Aims of the proposed research are to discover the reasons for the less than optimal immune reconstitution in a number of these infants. Aim 1) To characterize the phenotypes and functions of T cells longitudinally after allogeneic stem cell therapy in infants with SCID. Thymic output will be monitored by TRECs and T cell diversity by Immunoscope analyses. The hypotheses to be tested are: 1) that the decreased thymic output in the teenage years is due to the fact that the stem cells and the thymus are haploidentical to each other, 2) that this is not due to failure of engraftment of true hematopoietic stem cells and 3) that the absence normal peripheral lymphoid tissues may affect the T cell repertoire diversity as well as the types of cytokines they produce. Aim 2) To examine why some patients engraft donor B cells and others do not and to discover why host B cells in certain types of SCID produce immunoglobulins normally while others do not. The hypotheses to be tested are: 1) that GVHD facilitates donor B cell engraftment, 2) that this is contributed to by transplacental transfer of maternal lymphocytes, 3) that NK cell development has no effect on B cell function, 4) that the underlying mutation determines the ability of the host B cell to function and 5) that the lack of FDC networks, class switch and antibody response may be related to altered LT expression and/or function resulting from that mutation. Aim 3) To examine whether host dendritic cell abnormalities contribute to inadequate B and NK cell functions in X-linked and Jak3-deficient SCIDs. The hypothesis to be tested is that the host dendritic cells cannot function due to defects caused by the underlying mutations in these types of SCID and that donor-derived NK cells and functions will decline with time due to abnormal IL-15-dependent host dendritic cell (DC) functions. The function of blood monocyte-derived DC will be assessed by measuring DC IL-12 release (following stimulation with LPS, anti-CD40, poly I:C, CpG, IL-4, GM-CSF, and IFNgamma) and DC capacity to activate allogeneic T cells in mixed lymphocyte responses.